A. Field of the Invention
The present invention relates to a method of and machine for writing a pattern the surface of a substrate by an electron beam.
B. Background of the Invention
The writing of patterns on substrates, for example circuits on wafers, by electron beam lithography is subject to closely controlled parameters to ensure accurate writing of pattern features in conjunction with a high throughput rate. The beam current downstream of a final focussing stage, i.e. probe current, is set in dependence on a number of factors, including accelerating voltage for producing the beam, beam spot size on the substrate surface, rate of step of the beam between successive writing positions during scanning of the substrate surface, i.e. dose clock or exel clock, sensitivity of an electron-sensitive resist on the surface, and correction needed for the proximity of features of the pattern. Proximity correction is required because the electron doses in adjoining areas can generate mutual influencing by the backscatter effect of secondary electrons and this leads to impairment of the writing accuracy. The doses need to be less in such areas and also in corner areas and inner areas of pattern shapes. Accordingly, constant alteration of dose has to be carried out during writing.
Various methods are at present employed for the purpose of changing dose. A first method is to vary the step rate or dose clock. This method has the disadvantage of reducing throughput, as part of the pattern then has to be written below the maximum rate. The maximum rate can otherwise be quite readily achieved by an electron beam machine equipped to provide a high density of the probe current. A second method is to change the probe current by altering one or more of the factors determining beam spot size or focus. This method, too, has the disadvantage of reducing throughput, due to the time taken to make the alteration. The alteration time may amount to several seconds and this time is lost whenever a dose change is required.
In W094/28574 there is disclosed a third method, which employs variation in electron dose level for particular pixels between 100 percent and two lesser values. Inner regions of a pattern are written with maximum dose and edge regions with a selectable one of the lesser doses, the dose variation being achieved by beam modulation under the control of blanking pulses of different widths. This method serves the purpose of defining the position of the edges of written patterns, in particular by displacing the edges relative to grid lines of a scanning raster, and is not concerned with dose control for any other corrective purpose. A similar method is disclosed in Murray et al, xe2x80x98Experimental evaluation of an electron-beam pulse modulated blanker (160 MHz) for next-generation electron-beam raster scan systemsxe2x80x99 in Journal of Vacuum Science and Technology B, Vol. 13, No. 6 (November/December 1995), which describes use of three, rather than just two, sub-maximum dosage levels for writing pattern edges.
It is therefore the object of the invention to provide a method and a machine by which, in the course of pattern writing by an electron beam, dose rate may be varied as required for specific problematic parts of the pattern without loss in writing throughput time.
Other objects and advantages of the invention will be apparent from the following description.
According to a first aspect of the present invention there is provided a method of writing a pattern on the surface of a substrate by an electron beam, the method comprising the steps of exposing the substrate surface to an electron beam controlled to progressively describe the pattern by stepped movement of a focussed spot of the beam over the surface, and varying the exposure of the surface to the beam by selectably modulating the beam in the periods between successive movement steps to reduce the level of electron dose in predetermined positions of the beam spot on the surface, wherein said predetermined positions of the beam spot comprise positions associated with inner regions of feature shapes in the pattern, positions associated with pattern features of such proximity to one another as to be susceptible to mutual influencing during writing, and/or positions associated with linear pattern features liable to contain noise components when written, the writing of said linear pattern features by the modulated beam being repeated at least once to average the components.
The writing of the entire pattern is preferably carried out with a predetermined substantially constant rate of stepped movement of the beam spot, i.e. a substantially constant dose clock. This rate of movement is preferably the maximum rate still allowing sufficient exposure for writing of those features of the pattern requiring the highest level of dose. In that case, the beam modulation allows the dose level to be varied as required, so that the pattern can be written at the maximum step rate established by reference to the particular pattern shapes, the substrate resist sensitivity and the various beam parameters. No change in the step rate or other beam parameters is then necessary for the purpose of dose variation.
The predetermined positions of the beam spot where beam modulation is carried out are positions in which the dose level required for writing of the associated pattern parts is less than that required for the part requiring the highest level, namely positions associated with inner regions of feature shapes in the pattern, where a lesser dose is needed, and/or those associated with pattern features of such proximity to one another that mutual influencing, such as from backscattered secondary electrons, might occur during writing, and/or those associated with linear pattern features liable to contain noise components when written. In the last-mentioned case, the features can be written repeatedly by the modulated beam so as to average the noise components.
The beam modulation is preferably carried out substantially without change in the beam spot size or displacement of the beam spot during the writing. The modulation is preferably between maximum dose and zero dose, thus with complete blanking of the beam. This can be achieved by, for example, deflection of the beam to and from a blanking surface. The rate of stepped movement of the beam and the rate of modulation of the beam can be in any desired ratio, such as a ratio in which the dose is reduced by substantially one half by the beam modulation.
According to a second aspect of the invention there is provided an electron beam pattern writing machine for carrying out the method of the first aspect of the invention, the machine comprising beam generating and focussing means for generating an electron beam and focussing the beam as a spot on a surface of a substrate on which the pattern is to be written, control means for controlling the beam to progressively describe the pattern while moving the beam spot in steps over the substrate surface, and modulating means for selectably modulating the beam in the periods between successive movement steps to reduce the level of electron dose in predetermined positions of the beam spot on the surface, wherein said predetermined positions of the beam spot comprise positions associated with inner regions of feature shapes in the pattern, positions associated with pattern features of such proximity to one another as to be susceptible to mutual influencing during writing, and/or positions associated with linear pattern features liable to contain noise components when written, the writing of said linear pattern features by the modulated beam being repeated at least once to average the components.
The modulating means preferably comprises electrostatic deflecting elements extending parallelly to the beam, the elements being actuable by, for example, bipolar switching amplifier means. When the stepped movement of the beam spot is controlled by the control means to be at a substantially constant rate; the rise and fall times of the amplifier means are preferably less than that rate.